The invention relates to a method for reducing the noise caused by vehicle tires of passenger cars, trucks, and the like rolling on a surface, such as a road surface or a runway surface.
It is a fact that tire noise contributes substantially to the traffic noise. It has been found that at a speed of about 50 km/hour the engine noise and the tire noise are approximately equal to each other. However, when the speed is increased the tire noise rises substantially and exceeds the engine noise so that the tire noise becomes predominant. Relatively little has been done heretofore in order to alleviate or avoid this problem. Practical approaches to this problem heretofore aimed at reducing tire noise emissions by respectively designing the external tire contour, for example, by providing the tire tread with an uneven pitch. This type of tire tread has reduced the apparent tire noise to some extent. However, the apparent reduction is not so much in an absolute diminishing of the noise generated, but rather in shifting the frequency of the generated noise into a range which is apparently less bothersome. Efforts to actually reduce the body noise emission and the so-called "air pumping" have met with little success heretofore.
Our investigations in this area have shown that for this type of noise another generating mechanism is controlling. It has been found that due to the irregularities in the tire itself the repeated rolling movement causes nonstationary movement of the tire walls. Such tire wall movement simultaneously radiates noise outwardly of the tire and also into the space defined by the tire and filled with air under pressure. Due to the higher air density and due to the space resonances the radiation resistance inside the tire is substantially larger than outside thereof, whereby substantially more noise energy is transmitted into the tire than outwardly. Further, due to the small noise absorption inside the tire a noise level increase or amplification, so to speak, is provided inside the tire. As a result, the so-called internal tire noise is radiated outwardly through the sidewalls of the tire surfaces which have a smaller attenuation than the tire tread. The relatively high frequency humming tone of truck tires, for example, is due to the just explained fact that the tire sidewalls have a lower noise insulation quality than the tire tread.